Instances arise when the same physical carrier is to be employed for the joint transmission of two data streams significantly differing in their bit rates, namely a primary bit stream of relatively high pulse cadence or repetition frequency--corresponding to a short cycle length--and a secondary bit stream of relatively low cadence and large cycle length. The secondary bit stream may contain, for example, digitized voice samples in a telecommunication system serving for telephone traffic and high-speed data transmission; that bit stream could also carry supervisory information relating to the free or busy state of a line, the address of a calling or called subscriber, the existence of a malfunction and so on. With metallic conductors serving as the physical carrier, a line code utilized for the primary data stream will generally occupy a limited band of the available frequency spectrum, allowing other portions of that spectrum to be used for the secondary data flow; that part of the spectrum may range, for example, between zero and a frequency on the order of 1/100 of the bit frequency of the primary stream. In a coaxial cable, moreover, speech transmission may be confined to a single pair of copper wires while other conductors convey the primary data flow.
When the carrier is an optical fiber, different bands of the frequency spectrum cannot be readily separated by filtering. The use of metallic wiring alongside such a fiber becomes ever less economical as improvements in fiber-optical technology enable a wider spacing of signal repeaters or regenerators along the line. Such conductors also tend to create interferences due to stray currents and may attract electrical discharges in a thunderstorm.
The use of a separate second fiber for the additional data flow is also uneconomical since both fibers would have to undergo signal regeneration in the associated repeater stations.
There is further known a technique termed "bit insertion" which involves a multiplexing of the two data streams with resulting increase in the overall bit frequency. Moreover, the equipment needed for combining and separating these interleaved bit streams is rather cumbersome and expensive.